• ESLint Version: 4.15.0
• eslint-plugin-vue Version: 4.2.0
• Node Version: 8.9.3

"vue/valid-v-on": 1,

<template>
  <input @keyup.arrow-down="foo">
<!--        ^^^ [eslint-plugin-vue]
                [vue/valid-v-on]
                'v-on' directives don't support the modifier 'arrow-down'. 
-->
</template>

Since vue ^2.5.0 it's possible to use valid modifiers from KeyboardEvent.key.
It would be nice to expand the list of them at least for the most used ones.